1. Field of the Invention
This invention relates to apparatus for coating a soluble coating material upon a substrate, and more specifically to achieve high throughput, high uniformity, and small quantity of the soluble material used.
2. Description of Related Art
Photoresist, or resist henceforth, is usually deposited to a substrate, or more particularly a wafer, by means of forming a puddle followed by spinning. A large puddle of resist covering more than half of the substrate area is applied via a dispenser that directs a steady flow of resist in liquid form. The thickness of the puddle is of the order of a millimeter. The substrate is then spun at a speed ranging from 1,000 to 10,000 rpm to thoroughly spread out and remove the excess resist to result in a film thickness on the order on between a fraction of micrometer and a few micrometers. Therefore, only a small percentage of the resist material actually remains on the substrate. Most of the resist material dispensed is wasted, resulting in high cost and waste disposal problems.
U.S. Pat. No. 3,695,911 of Jerry Polin et al for "Method for Applying a Flowable Substance to a Workpiece", Oct. 3, 1972, teaches depositing a quantity of the liquid substance on the surface of the substrate while the substrate is at rest, and then rapidly rotating the substrate at a high rate of acceleration to form a layer of uniform thickness. This prior art does not teach depositing and spinning in a solvent-vapor-rich environment. Without it, a large amount of liquid substance is required to wet a large area on the substrate to prevent the onset of drying which produces film non-uniformity.
U.S. Pat. No. 4,640,846 of Yue Kuo for "Semiconductor Spin Coating Method", Feb. 3, 1987, mentions depositing the liquid resist material semi-uniformly by spraying or other methods. The spinning does not take place in a solvent-vapor-rich environment. Therefore, the specific thin-coating capability of spray coating is not exploited. That is, even if a thin semi-uniform pre-spin coating were made by spraying, a non-uniform finished coating results because of partial drying of the liquid before and during spin, when a solvent-vapor-rich environment is not provided. Spraying a sufficiently thick layer, as required by this invention to keep the liquid from drying, results in wasting the liquid material.
U.S. Pat. No. 4,290,384 of Ausschnitt et al for "Coating Apparatus", Sep. 22, 1981, attempts to reduce material consumption by converting the coating material to a mist by means of an ultrasonic nebulizer and transported by a carrier gas to a chamber which contains a stationary substrate. The mist is allowed to settle onto the substrate under the force of gravity. Spin coating is not used. The final thickness and its uniformity are extremely sensitive to the liquid temperature, ultrasonic drive power, concentration of the liquid, and the volume delivered; thus difficult to control.
U.S. Pat. No. 4,347,302 of Alexander Gotman for "Process for Applying a Thin Coating in Liquid Form and Subsequently Drying It", Aug. 31, 1982, teaches the planarizing advantage of spinning in a solvent-vapor-rich environment. The substrate is moved into a large vapor chamber in an airtight substrate carrier which is essentially a smaller vapor chamber. Then, the carrier is opened to expose the top of the substrate to receive the liquid resist material and to spin it off to achieve the desired thickness and uniformity. After spinning, the substrate is moved to a vapor-free area and a baking station in the same airtight substrate carrier. Spray depositing is not used. Hence, material consumption is not reduced. Another disadvantage is that the substrate carrier is complicated. In addition, it requires mechanical clamping instead of vacuum clamping to fasten the substrate to the substrate stage, which, subjects the substrate to a high probability of defects and damage. Furthermore, transporting the carrying chamber is wasteful in space and processing time.
U.S. Pat. No. 5,094884 of Hillman and Mohondro for "Method and Apparatus for Applying a Layer of a Fluid Material on a Semiconductor Substrate." Teaches using a nozzle to produce a rectangular or oblong liquid dispensing pattern. Neither spraying nor a solvent-vapor-rich environment is used. Reduction of material consumption cannot be achieved.
U.S. patent application Ser. No. 07/784,290 filed Oct. 29, 1991 of Cardinali and Lin for "A Material Saving Resist Spinner and Process" describes saving resist by spray coating a thin layer of resist on the substrate which is kept in a solvent-vapor-rich environment followed with spinning in the same environment. This prevents the resist from drying so that the spin-off can produce a uniform coating even though the starting thickness is only slightly larger than the final thickness. In addition to the spray-coating and solvent-vapor-rich requirements, another key requirement of the prior invention is a delay between spray coating and spinning to permit the resist to reflow evenly, without which a uniform coating cannot be achieved. The delay or spread-out time is on the order of 30 seconds. It significantly reduces throughput.
U.S. patent application Ser. No. 08/100,311 filed Aug. 2, 1993 of Lin for "A high-throughput low-consumption resist coating device" teaches small-particle, low-velocity spraying, temperature regulations, and vapor evacuation control to increase the throughput and uniformity of the spray-and-spin-in-vapor technique. The resist material is delivered to the substrate surface by means of small-particle, low-velocity spraying in the presence of a rich vapor.